Luna 4 was the USSR's first successful spacecraft of their "second generation" lunar program. The spacecraft, also referred to as an automatic interplanetary station, launched on a Molniya 8K78 on 2 April 1963 at 08:04:00 UT (A launch time of 08:16:37 UT has also been reported) Rather than being sent on a straight trajectory toward the Moon, the spacecraft was placed first in a 167 x 182 km Earth orbit and then was rocketed in a curving path towards the Moon. Luna 4 achieved the desired initial trajectory but during trans-lunar coast the Yupiter astronavigation system failed (most likely due to thermal control problems) and the spacecraft could not be oriented properly for the planned midcourse correction burn. Communications were maintained, but Luna 4 missed the Moon by about 8400 km (sources give reports of 8336.2, 8451, and 8500 km) at 13:25 UT on 5 April 1963 and entered a 89 250 x 694 000 km equatorial Earth orbit. The spacecraft transmitted at 183.6 MHz at least until 7 April. The orbit is believed to have been later perturbed into a heliocentric orbit. Luna 4 had a mass of 1422 kg and carried an imaging system and radiation detector. The structure was built by design bureau OKB-1 based on the Ye-6 satellite body. The intended mission of the probe was never revealed, it was announced it would travel to "the vicinity of the Moon". It was speculated the probe was designed to perform a soft landing on the Moon based on the trajectory and on the later attempted landings of the Luna 5 and 6 spacecraft, as well as the advances made over the 3 years since the successful Luna 3 flyby. (And the fact that a lecture program entitled "Hitting the Moon", scheduled to be broadcast on Radio Moscow at 7:45 p.m. the evening of April 5, was cancelled.)